A‐DA’D‐A Type Pentacyclic Small Molecule Acceptors to Exceed 16.5% Efficiency by Heteroatom Effect at the Outer Side Chain

Comprehensive Summary In this work, we adopt a “heteroatom side‐chains” modification strategy to modify the thiophene units in A‐DA'D‐A (acceptor‐donor‐acceptor’‐donor‐acceptor) type pentacyclic SMAs (small molecule acceptors), that is, introducing branched alkyl chain at the β‐position of thiophene instead of straight alkyl chain, and then introducing oxygen atom at the third‐position on the basis of branched chain. Two new pentacyclic SMAs (BZ4F‐EH and BZ4F‐OEH) were synthesized, and the influence of the heteroatom side‐chains on photoelectric properties of A‐DA'D‐A type pentacyclic SMAs was systematically studied. Compared with our previously reported BZ4F (Y26), BZ4F‐EH shows slightly blue‐shifted absorption, while BZ4F‐OEH has obvious red‐shifted absorption. As a result, BZ4F‐OEH‐based binary device achieved a high power conversion efficiency (PCE) of 16.56% with a fill factor (FF) of 79.3%, which is the highest efficiency of pentacyclic SMAs to date. The results indicated that the modification of hetero‐atomization on the β‐position of thiophene is a simple and efficient way to improve the PCE of A‐DA'D‐A type SMAs, and this work provides an important guideline for future molecular design. We utilized the molecular backbone based on our previously reported pentacyclic acceptor BZ4F (Y26) due to its simple molecular skeleton. Two SMAs were designed and synthesized with 2‐ethylhexyl (BZ4F‐EH) and 2‐((2‐ethylhexyl)oxy)ethyl (BZ4F‐OEH). The introduction of branched alkyl chain instead of straight alkyl chain increases the PCE by 2.9%. However, based on the branched alkyl chain, the introduction of the third‐position oxygen atom in the outer side chain increases the PCE by 12.9%.